The efferent cochlear projections of the superior olivary complex in the mustached bat

Following the placement of horseradish peroxidase in the scala tympani, labeled neurons were found in the ipsilateral interstitial nucleus (INT) and throughout the ipsilateral and contralateral dorsomedial periolivary nuclei (DMPO). The neurons in the INT were morphologically distinct from those in the DMPO. The INT neurons formed a thin shell over the lateral superior olivary nucleus (LSO) and their dendrites extended into the body and hilar region. The DMPO neurons had long, tapering dendrites that extended in every direction. Data indicate that the crossed fibers in the floor of the ventricle arise entirely from the DMPO while uncrossed olivocochlear fibers originate in the INT and DMPO. It was estimated that 75% of the efferent fibers arise from the INT and 25% from the DMPO. Approximately 70% of the efferent neurons in each DMPO project to the contralateral cochlea via the crossed olivocochlear bundle. The number of olivocochlear neurons associated with each ear was determined to be approximately 1585. This number is similar to that found in cats and guinea pigs, but the number of neurons per unit length of the basilar membrane is considerably higher in the mustached bat than in other species examined to date. The compact, restricted locations of the neurons in the INT and DMPO in the mustached bat are different from those described for most other mammals and the arrangement in the mustached bat offers advantages over other species for future anatomical and physiological studies.